Water-Insoluble Polyglutamic Acid Fibers

ABSTRACT

A water-insoluble polyglutamic acid (PGA) fiber and a preparation method thereof are provided. In the preparation method, the PGA is cross-linked by a cross-linking agent and then passes through a spinning nozzle to form PGA fibers. Therefore, the highly water-absorbing PGA, which cannot be spun by conventional methods, can be spun to form PGA fibers and maintain the high water-absorption ability.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. application Ser. No.12/757,288, filed on Apr. 9, 2010, which was based on, and claimspriority to Chinese application serial No. 200910137040.X, filed Apr.21, 2009, the full disclosures of which are incorporated herein byreference.

BACKGROUND

1. Technical Field

The disclosure relates to a method of preparing water-insoluble fibersfrom an extremely hydrophilic polypeptide, which is very soluble inwater. More particularly, the disclosure relates to a water-insolublepolyglutamic acid fibers and a preparation method thereof.

2. Description of Related Art

There are two kinds of commonly used materials with highwater-absorption ability. One is based on acrylic acid, and the other isbased on carbohydrate. For the one based on acrylic acid, the materialis low-cost and has significant water-absorption ability, but it is notbiodegradable and will therefore cause environmental problem. For theone based on carbohydrate, such as starch, chitin, sodium alginate, andcarboxymethyl cellulose (CMC), the material is low-cost andbiodegradable, but its water-absorption ability is limited. Therefore,it is needed to develop a material that has significant water-absorptionability and biodegradability.

It is well known that polyglutamic acid (PGA) has high water-absorptionability and high biodegradability, and PGA is also very soluble inwater. Therefore, PGA can be easily dissolved once if PGA gets in touchwith water or even only exposes to water vapor in the air. This is whyPGA without any treatment cannot be used as water-absorption materialand hence cannot be spun into fibers. Conventionally, PGA iscross-linked to have its conformation maintained and also to avoid beingdissolved.

The methods of cross-linking PGA include physical methods and chemicalmethods. In physical methods, e.g. JP Publication No. 6-322358, the PGAis cross-linked by γ-ray to produce hydrogel with high water-absorptionability. However, the γ-ray equipment is complicated and expensive.Therefore, this method is not suitable for industry.

In chemical methods, a cross-linking agent is used to performcross-linking reaction on PGA. Conventional cross-linking agents containfunctional groups of dialdehyde, diamine, or diepoxide. For example, JPPublication No. 11-343339 discloses a method of isolating a highconcentration γ-PGA from a culture broth, and using the isolated γ-PGAas the starting material for the cross-linking reaction with a diepoxycompound to obtain a biodegradable, water absorbable hydrogel. In U.S.Pat. No. 6,998,367, a cross-linking agent having dialdehyde, diamine, ordiepoxide functional groups and a metal ion were used to cross-link PGAto produce water absorbable material that is water-insoluble. In U.S.Pat. No. 7,125,960, glutaraldehyde, ethylene glycol diglycidyl ether,and carbodiimide were used to cross-link PGA to obtain water absorbablegel.

The methods above have disclosed that water-insoluble PGA gel can beobtained by cross-linking technique, but the water-absorption ability ofPGA was largely decreased. Hence, the biomedical applications of PGA arelimited. Accordingly, if PGA can be spun into water-insoluble fibers,the above mentioned problem can be solved. However, the unmodified PGAis very soluble in water, and hence conventional spinning methods cannotbe used to spin PGA fibers. Although the available techniques are ableto produce water-insoluble PGA, only formation of PGA gel was disclosed.Nothing about how to form PGA fibers by spinning technique wasdisclosed. Moreover, the PGA gel does not provide sufficient flowabilityand hence cannot be spun by conventional spinning methods.

SUMMARY

Accordingly, a method of preparing water-insoluble polyglutamic acid(PGA) fibers and PGA fibers made by the same method is provided to solvethe prior problems.

According to an embodiment of this invention, the method of spinningwater-insoluble PGA fibers comprises the following steps. First, a PGAaqueous solution having an initial viscosity of no is prepared. Then, across-linking agent is added to the PGA aqueous solution to perform PGAcross-linking reaction. Next, the PGA cross-linking reaction isperformed until a spinnable viscosity is reached to form PGA fibers byspinning out the cross-linked PGA solution from a spinning nozzle,wherein the spinnable viscosity is from [η₀+ 1/500(η_(f)−η₀)] to <η_(f).Finally, the PGA fibers are dried to perform post-cross-linking reactionto completely react the residual cross-linking agent.

The water-insoluble PGA fibers can be made by the above method.

It is to be understood that both the foregoing general description andthe following detailed description are by examples, and are intended toprovide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a process flow of preparing polyglutamic acid fibers accordingto an embodiment of this invention.

DETAILED DESCRIPTION

In the following detailed description, for purposes of explanation,numerous specific details are set forth in order to provide a thoroughunderstanding of the disclosed embodiments. It will be apparent,however, that one or more embodiments may be practiced without thesespecific details. In other instances, well-known structures and devicesare schematically shown in order to simplify the drawing.

Since polyglutamic acid (PGA) has high water-absorption ability and highwater solubility, PGA has to be shaped by cross-linking treatment forvarious applications. Although some cross-linking treatments of PGA havebeen disclosed, all the obtained PGA is in the forms of gel, grain, ormembrane. PGA in the form of fiber has not been disclosed yet.

Any person skilled in the art can understand that the super-highwater-absorption ability of the PGA leads spinning the PGA fibersimpossible if the PGA is not modified or processed at all, since it ishard to maintain the conformation of the PGA fibers during theconventional spinning process. Moreover, even if the PGA fibers can beobtained, the PGA fibers will be dissolved after absorbing water vaporwhen the PGA fibers are exposed in the air. Furthermore, if across-linking agent is added into the PGA solution to cross-link thePGA, the viscosity of the PGA solution is often too high to be spunbecause the cross-linking reaction is overly performed. Therefore, it isdifficult to control the cross-linking degree of the PGA solution tomake the spinning of PGA fibers possible and still maintain theconformation of the PGA fibers at the same time.

Accordingly, a method of spinning water-insoluble PGA fibers isprovided. FIG. 1 is a process flow of preparing polyglutamic acid fibersaccording to an embodiment of this invention.

In step 102, a PGA aqueous solution (abbreviated as PGA solution below)is prepared. The method of preparing the PGA solution has no speciallimitations. Any method that can prepare the PGA solution can be used.For example, PGA or its sodium form can be directly added into purewater. Then, the solution is stirred until the PGA or its sodium form iscompletely dissolved in the water to reach the desired PGAconcentration. Since it is hard to spin the PGA fibers when the solidcontent of the PGA solution is too low or too high, wherein the solidcontent is the residual solid substance after removing the liquidsubstance of the PGA solution. Therefore, the PGA concentration of thePGA solution is 6-30 wt % according to an embodiment and more preferably6-25 wt % according to another embodiment.

In step 104, a cross-linking agent is added into the PGA solution tostart the cross-linking reaction of the PGA. Since the PGA has many freecarboxylic acid groups (—COOH) to be cross-linked, the cross-linkingagent is better to has at least two functional groups that can reactwith the free carboxylic acid groups. Therefore, the cross-linking agentcan be a compound that has at least two functional groups of epoxy,amine, aldehyde, or double-bound, for example.

The cross-linking agent having at least two epoxy groups includes, butis not limited to, polyglycidyl ether, such as ethylene glycoldiglycidyl ether, diglycerol polyglycidyl ether, polyglycerolpolyglycidyl ether, or polyoxyethylene sorbitol polyglycidyl ether.

The cross-linking agent having at least two amine groups includes, butis not limited to, ethylene diamine, which can be referred to U.S. Pat.No. 5,279,821.

The cross-linking agent having at least two aldehyde groups includes,but is not limited to, glutaraldehyde, which can be referred to U.S.Pat. Nos. 6,699,367 and 7,125,960.

The cross-linking agent having at least two double-bond groups includes,but is not limited to, N,N′-(1,2-dihydroxyethylene).

If the added amount of the cross-linking agent is not enough to maintainthe conformation of the PGA fibers, the PGA fibers will be dissolvedimmediately after absorbing water vapor in the air. However, if theadded amount of the cross-linking agent is too much, the cross-linkingdegree of the PGA solution will be too high to spin the PGA solution. Inaddition, the excessive cross-linking agent is also a waste. Therefore,the ratio of the added equivalent of the cross-linking agent over theequivalent of the PGA's carboxylic acid groups is 1:45-1:1 according toan embodiment, and 1:15-1:1 according to another embodiment.

In step 106, after adding the cross-linking agent, the cross-linkingagent can immediately react with PGA to start the cross-linkingreaction. The cross-linking reaction can be performed even under roomtemperature. Therefore, the reaction temperature has no speciallimitations. However, in view of the commercial production, the reactionrate can be increased by heating. Since PGA is a polypeptide molecule,PGA can be denatured or decomposed at high temperature. Therefore, thecross-linking temperature is 35-85° C. according to an embodiment, and45-75° C. according to another embodiment.

In order to increase the rate of the cross-linking reaction, any personsskilled in the art can understand that stirring the PGA solutioncontaining the cross-linking agent can be further used to increase thecross-linking rate. The PGA solution containing the cross-linking agentcan be stirred by a stir rod, for example.

It is necessary to cross-link PGA to convert the water-soluble PGA intowater-insoluble PGA. However, if PGA is over cross-linked, the viscosityis often too high to spin out PGA fibers from a spin nozzle. It is foundin this invention that PGA can be spun after being partiallycross-linked. In this case, the PGA fibers will not be dissolvedimmediately after being exposed to the air. Subsequently, the postcross-linking reaction can be performed on the PGA fibers to reduce itswater solubility and hence mechanical strength can be improved.

Since the viscosity of the PGA solution will be increased with theincrease of the cross-linking degree of the PGA. When the viscosity ofthe PGA solution is increased to a certain value, the partiallycross-linked PGA in the solution starts to climb the stir rod. Thephenomenon is called “rod climbing.” Accordingly, it is assumed that theinitial viscosity of the PGA solution is η₀, and the viscosity of thePGA solution starting to “climb rod” is η_(f). Therefore, in step 108,the spinnable viscosity of the PGA solution is [η₀+ 1/500(η_(f)−η₀)] to<η_(f) according to an embodiment, and [η₀+¼(η_(f)−η₀)] to[η₀+¾(η_(f)−η₀)] according to another embodiment.

The spinning method for spinning the PGA fibers can be any availablespinning techniques such as dry spinning, wet spinning, orelectrospinning, etc. Any person skilled in the art knows that wetspinning technique utilizes a coagulant bath to solidify fibers, but dryspinning and electrospinning do not need coagulant bath. Therefore, whenwet spinning is performed to spin PGA fibers, the coagulant bath usuallycontains an organic solvent which is preferably alcohol, ether, ketone,or any combinations thereof that has 1−5 carbons. The alcohol above canbe, but not limited by, methanol, ethanol, or iso-propanol, for example.The ether above can be, but not limited by, dimethyl ether, diethylether, or any combinations thereof, for example. The ketone above canbe, but not limited by, acetone, for example. Since a person skilled inthe art understands the related techniques about the wet spinning, andthe related technical details can be determined by himself/herselfaccording to this disclosure, the related technical details are omittedhere.

Moreover, the cross-linking reaction will proceed until the reaction iscompleted, and the wet spinning needs time to complete the spinningprocess for all of the partially cross-linked PGA solution. Therefore,it is better to reduce the reaction rate to prevent the viscosity of thePGA solution from being too high to pass the spin nozzle. One skilled inthe art can understand that if a continuous production process is usedto cross-link and spin out the PGA fibers, it may not need to reduce thecross-linking reaction rate.

The method for reducing the reaction rate can be any available methodswithout special limitations. For example, the reaction temperature ofthe PGA solution having the spinnable viscosity can be decreased todecelerate the cross-linking reaction. The reaction temperature can bedecreased to 4-10° C. or 6° C. to effectively decrease the reaction ratewithout solidifying the PGA solution, for example. The temperature canbe decreased by keeping the PGA solution in an ice bath or circulatingcooling water in the wet spinning step.

According to another embodiment, a retardant agent can be added todecrease the cross-linking reaction rate. The retardant agent can be,but not limited by, D-glutamic acid.

In step 110, the PGA fibers are dried to facilitate the cross-linkingreaction to be proceeded in the PGA fibers, i.e. the post-cross-linkingreaction and remove the excessive water to strengthen the PGA fibers'structure. If the drying temperature is too high, some side reactions,such as PGA decomposition reaction may occur. If the drying temperatureis too low, the rate of the post-cross-linking reaction will be tooslow. Therefore, the drying temperature can be 25-90° C. according to anembodiment, 35-85° C. according to another embodiment, and 45-75° C.according to yet another embodiment, for example. The drying time can be20 hours, for example.

The embodiments below illustrate the spinning method of the PGA fibersand the tests made for the PGA fibers. These illustrations below are notused to limit the claim's scope, any ones skilled in the art can easilymade modifications and variations which are covered by the claims ofthis invention.

Preparation of PGA Fibers EMBODIMENT 1

Sodium polyglutamate (from Vedan Corp.) was mixed with water to preparea 6 wt % of PGA solution. The initial viscosity of the PGA solution was56.4 cp.

Next, a cross-linking agent, ethylene glycol diglycidyl ether (fromTOKYO YASEI), was added into the PGA solution. The added amount of theethylene glycol diglycidyl ether was 7 μL/100 g PGA solution, which wasequal to ¼ equivalent of ethylene glycol diglycidyl ether per equivalentof PGA. After adding the cross-linking agent, the PGA solution wasstirred at a speed of 50 rpm to have the cross-linking reactionperformed under 60° C. If the cross-linking reaction was not interrupteduntil the time of “rod climbing,” which took about 5.5 hours, therod-climbing viscosity was 997.2 cp.

The spinning test was performed when the viscosity was increased to 82cp (about 240 minutes) to let the PGA solution pass the spinning nozzlefor obtaining PGA fibers. In order to prevent the PGA solution frombeing over cross-linked before passing the spinning nozzle, thetemperature of the remainder PGA solution was kept as low as 6° C.

The obtained PGA fibers were then solidified by passing throughiso-propanol (from ECHO chemical, Model No. TG-078-000000-75NL). Aftercollecting solidified PGA fibers, the solidified PGA fibers were driedin an oven at a temperature of 60° C. for about 20 hours to successfullyobtain the final PGA fibers.

EMBODIMENT 2

The operation conditions of Embodiment 2 were about the same as those ofEmbodiment 1. The only difference was the concentration of the initialPGA solution was 10 wt %, and the added amount of ethylene glycoldiglycidyl ether was 5 μL/100 g PGA solution, which was equal to 1/9equivalent of ethylene glycol diglycidyl ether per equivalent of PGA.

The initial viscosity of the PGA solution was 60.2 cp, and therod-climbing viscosity was 1247.4 cp (cross-linked for about 2.3 h). Inthis Embodiment, the spinning started when the viscosity of PGA solutionwas 92.3 cp (about 40 minutes). The PGA fibers can be successfullyobtained.

EMBODIMENT 3

The operation conditions of Embodiment 3 were about the same as those ofEmbodiment 1. The only difference was the concentration of the initialPGA solution was 15 wt %, and the added amount of ethylene glycoldiglycidyl ether was 5 μL/100 g PGA solution, which was equal to 1/14equivalent of ethylene glycol diglycidyl ether per equivalent of PGA.

The initial viscosity of the PGA solution was 74.6 cp, and therod-climbing viscosity was 964.8 cp (cross-linked for about 2.2 h). Inthis Embodiment, the spinning started when the viscosity of PGA solutionwas 152.1 cp (about 90 minutes). The PGA fibers can be successfullyobtained.

EMBODIMENT 4

The operation conditions of Embodiment 4 were about the same as those ofEmbodiment 1. The only difference was the concentration of the initialPGA solution was 20 wt %, and the added amount of ethylene glycoldiglycidyl ether was 6 μL/100 g PGA solution, which was equal to 1/15equivalent of ethylene glycol diglycidyl ether per equivalent of PGA.

The initial viscosity of the PGA solution was 87.8 cp, and therod-climbing viscosity was 1124 cp (cross-linked for about 1.9 h). Inthis Embodiment, the spinning started when the viscosity of PGA solutionwas 185.3 cp (about 60 minutes). The PGA fibers can be successfullyobtained.

EMBODIMENT 5

The operation conditions of Embodiment 5 were about the same as those ofEmbodiment 1. The only difference was the concentration of the initialPGA solution was 20 wt %, and the added amount of ethylene glycoldiglycidyl ether was 2 μL/100 g PGA solution, which was equal to 1/45equivalent of ethylene glycol diglycidyl ether per equivalent of PGA.

The initial viscosity of the PGA solution was 83.7 cp, and therod-climbing viscosity was 1012.5 cp (cross-linked for about 2.6 h). Inthis Embodiment, the spinning started when the viscosity of PGA solutionwas 133.5 cp (about 60 minutes). The PGA fibers can be successfullyobtained.

EMBODIMENT 6

The operation conditions of Embodiment 6 were about the same as those ofEmbodiment 1. The only difference was the concentration of the initialPGA solution was 25 wt %, and the added amount of ethylene glycoldiglycidyl ether was 3 μL/100 g PGA solution, which was equal to 1/40equivalent of ethylene glycol diglycidyl ether per equivalent of PGA.

The initial viscosity of the PGA solution was 96.3 cp, and therod-climbing viscosity was 1221.9 cp (cross-linked for about 1.6 h). Inthis Embodiment, the spinning started when the viscosity of PGA solutionwas 225.6 cp (about 60 minutes). The PGA fibers can be successfullyobtained.

COMPARATIVE EMBODIMENT 1

The operation conditions of Comparative Embodiment 1 were about the sameas those of Embodiment 4, but the spinning started at 87.9 cp(cross-linked for only about 10 minutes), which was just a little bithigher than the initial viscosity, 87.8 cp. The result showed thatalthough the cross-linking degree was not enough, the PGA fibers werestill formed in the iso-propanol. However, once the PGA fibers left theiso-propanol and exposed in the air for only 30 seconds, the PGA fiberswere dissolved after absorption of water vapor in the air.

COMPARATIVE EMBODIMENT 2

The operation conditions of Comparative Embodiment 1 were about the sameas those of Embodiment 4, but the spinning started at 1130 cp(cross-linked for about 2 hours), which was higher than the rod-climbingviscosity, 1124 cp. Since the viscosity was higher than the rod-climbingviscosity, the PGA solution became an elastic solid. Therefore, the PGAsolution was hard to pass the spinning nozzle to obtain PGA fibers.

Tests for PGA Fibers

The tests of water absorbability and the weight loss after washing weredone for the obtained PGA fibers of Embodiments 1-6 to understandwhether the PGA fibers can maintain both of the high water absorptionability and the fiber conformation at the same time. The test resultsare shown in Table 1 below.

TABLE 1 The tests of water absorbability and the weight loss afterwashing for PGA fibers. Embodiment 1 2 3 4 5 6 water absorbability(weight 179.2 198.6 217.5 220.3 215.6 223.6 ratio) weight loss afterwashing 10.6 7.2 6.5 7.2 9.2 9.1 (wt %)

In the test of water absorption, PGA fibers were put in excessive amountof deionized water to sufficiently absorb water. Then, the surplus waterwas removed. The weights of PGA fibers before absorbing water and afterabsorbing water were both measured. The water absorbability wascalculated by the following formula (I):

(weight after absorbing water−weight before absorbing water)/weightbefore absorbing water   (I)

In the test of weight loss after washing, PGA fibers were put inexcessive amount of deionized water to sufficiently absorb water andthen stayed for 1 hour. Then, the surplus water was removed. Next, thePGA fibers were dried in an oven until the weight of PGA fibers remainsunchanged. The weights of PGA fibers before washing and after dryingwere both measured. The weight loss percentage was calculated by thefollowing formula (II):

(weight before washing−weight after drying)/weight before washing×100%  (II)

From the tested results of the water absorbability, it can be known thatthe PGA fibers of Embodiments 1-6 could still absorb water more than 150times of their weight. And more surprisingly, the conformation of thePGA fibers could maintain for at least one month without beingdecomposed.

From the tested results of the weight loss after washing, the weightlosses were all smaller than 15 wt % after the PGA fibers sufficientlyabsorbed water and stayed in water for 1 hour. The results show that themethod provided by this disclosure can solve the conventional problem tosuccessfully obtain “water-insoluble PGA fibers” with weight losssmaller than 15 wt % after absorbing water.

Accordingly, the spinning method of the PGA fibers and the obtained PGAfibers provided by this disclosure have completely solved theconventional problems of being unable to spin the PGA fibers andmaintain the conformation of the PGA fibers.

The reader's attention is directed to all papers and documents which arefiled concurrently with this specification and which are open to publicinspection with this specification, and the contents of all such papersand documents are incorporated herein by reference.

All the features disclosed in this specification (including anyaccompanying claims, abstract, and drawings) may be replaced byalternative features serving the same, equivalent or similar purpose,unless expressly stated otherwise. Thus, each feature disclosed is oneexample only of a generic series of equivalent or similar features.

1. Water-insoluble polyglutamic acid (PGA) fibers, which are made by: (a) preparing a PGA aqueous solution having an initial viscosity of no; (b) adding a cross-linking agent to the PGA aqueous solution to perform cross-linking reaction; (c) performing the cross-linking reaction until a spinnable viscosity is reached, wherein the spinnable viscosity is from [η₀+ 1/500(η_(f)−η₀] to <η_(f), and the η_(f) is the viscosity when the PGA aqueous solution begins to climb rod; and (d) forming PGA fibers by spinning out the cross-linked PGA solution from a spinning nozzle.
 2. Water-insoluble polyglutamic acid (PGA) fibers, which are made by: (a) preparing a PGA aqueous solution having an initial viscosity of no; (b) adding a cross-linking agent to the PGA aqueous solution to perform cross-linking reaction; (c) performing the cross-linking reaction until a spinnable viscosity is reached, wherein the spinnable viscosity is from [η₀+ 1/500(η_(f)−η₀)] to <η_(f), and the η_(f) is the viscosity when the PGA aqueous solution begins to climb rod; (d) reducing the rate of the cross-linking reaction; and (e) forming PGA fibers by spinning out the cross-linked PGA solution from a spinning nozzle.
 3. The Water-insoluble polyglutamic acid (PGA) fibers of claim 2, wherein the method of reducing the rate of the cross-linking reaction is decreasing the reaction temperature.
 4. Water-insoluble polyglutamic acid (PGA) fibers, which are made by: (a) preparing a PGA aqueous solution having an initial viscosity of η₀; (b) adding a cross-linking agent to the PGA aqueous solution to perform cross-linking reaction; (c) performing the cross-linking reaction until a spinnable viscosity is reached, wherein the spinnable viscosity is from [η₀+ 1/500(η_(f)−η₀)] to <η_(f), and the η_(f) is the viscosity when the PGA aqueous solution begins to climb rod; (d) forming PGA fibers by spinning out the cross-linked PGA solution from a spinning nozzle; (e) solidifying the PGA fibers by passing through a coagulant bath after the step (d); and (f) drying the PGA fibers to perform post-cross-linking reaction. 